Typically, a conventional electric linear motion actuator of this type includes a ball screw mechanism or a ball-ramp mechanism as a motion converter for converting rotary motion to linear motion.
Ball screw mechanisms and ball-ramp mechanisms used in such electric linear motion actuators convert rotary motion to linear motion utilizing a thread having a lead angle, and inclined cam surfaces, respectively, such that they can increase power to a certain extent. But these mechanisms cannot increase power to an extent required in an electric disk brake system.
Electric linear motion actuators including such a motion converter further include a speed reducer such as a planetary gear speed reducer to increase power, as disclosed in JP Patent publication 6-327190A. Such actuators tend to be large in size because they include an additional speed reducer.
The inventors of the present invention proposed, in JP Patent publication 2007-32717A and JP Patent publication 2007-37305A, electric linear motion actuators which can sufficiently increase power without using an additional speed reducer and which is suitable for use in an electric disk brake system, of which the linear motion stroke is relatively small.
The electric linear motion actuator disclosed in JP Patent publication 2007-32717A includes a housing, an outer ring member mounted in the housing, a rotary shaft extending along the center axis of the outer ring member and rotated by an electric motor, a carrier rotatable about the rotary shaft, and planetary rollers rotatably supported by the carrier between the radially outer surface of the rotary shaft and the radially inner surface of the outer ring member. A helical rib is formed on the radially inner surface of the outer member which is engaged in a plurality of circumferential grooves formed on the radially outer surface of each planetary roller at a pitch equal to the pitch of the helical rib. When the rotary shaft is rotated, the planetary rollers, which are in frictional contact with the rotary shaft, rotate about the center axes of the respective planetary rollers while revolving around the rotary shaft. This causes the carrier to move in one axial direction due to the engagement of the helical rib in the circumferential grooves. Thus, the carrier acts as an output member for linearly moving a member to be driven. The actuator disclosed in JP Patent publication 2007-37305A differs from the actuator disclosed in JP Patent publication 2007-32717A in that instead of the plurality of circumferential grooves, a helical groove is formed on each planetary roller.
Many of vehicle brake systems are hydraulic ones. But with an increase in sophisticated brake control schemes such as antilock brake control systems (ABS), electric disk brake systems are gathering attention these days because electric brake systems need no complicated hydraulic circuits.
As disclosed in JP Patent publication 2003-343620A, an electric disk brake system includes an electric motor, an electric linear motion actuator of the above-described type which is mounted in a caliper body, a member to be driven, and a member to be braked. When a brake pedal is depressed, the motor is actuated based on e.g. a signal, so that the linear motion actuator moves the member to be driven to press the driven member against the member to be braked, thereby applying a braking force to the member to be braked.
The electric linear motion actuators disclosed in JP Patent publication 2007-32717A and JP Patent publication 2007-37305A can sufficiently increase power without the need for an additional speed reducer and thus are compact in size. But since the carrier, which is linearly moved, is relatively short in axial dimension, if this type of electric linear motion actuator is used e.g. in an electric brake system, a tangential force applied to the braking member (driven member) from the member to be braked partially acts on the carrier as a lateral moment, making it difficult to smoothly and linearly move the carrier and the braking member.
After repeated use of this type of electric linear motion actuator, the radially outer surfaces of the rotary shaft and the planetary rollers as well as the radially inner surface of the outer ring member gradually become worn. When they become worn and the gaps between these members grow, the planetary rollers tend to incline in radial directions or incline in the circumferential direction (skew) while they are rotating about their axes and also revolving around the rotary shaft.
When the planetary rollers incline in radial directions or in the circumferential direction, the helical rib tends to be unevenly brought into contact with the circumferential grooves or the helical grooves, producing locally excessive loads on the contact portions between the helical rib and the circumferential grooves or helical grooves, which could partially break the helical rib or the circumferential grooves or helical grooves.
If the helical rib of the outer ring member and/or the circumferential grooves or helical grooves of the planetary rollers become worn, the contact pressure may become excessive between some turns of the helical rib and the circumferential grooves or helical grooves due to uneven contact therebetween, which could cause seizure and/or chipping at the contact portions.
Also, with this type of electric linear motion actuators, since the helical rib on the radially inner surface of the outer ring member has a different lead angle from the circumferential grooves or helical groove on the radially outer surface of each planetary roller, when the helical rib engages in the circumferential grooves or helical groove of each planetary roller with the radially outer surface of each planetary roller in rolling contact with the radially inner surface of the outer ring member, the shoulders of the helical rib tends to contact the edges of the grooves. The torque loss resulting from the contact resistance between the shoulders and edges reduces the efficiency of conversion to linear motion. The contact between the shoulders and edges could also cause wear of, or damage to, the shoulders and/or edges.